Method and system for coupling a light source assembly to an optical integrated circuit

ABSTRACT

Methods and systems for coupling a light source assembly to an optical integrated circuit are disclosed and may include a system comprising a laser source assembly having a laser, a rotator, and a mirror, where the laser source assembly is coupled to a die including an angled grating coupler and a waveguide. The system may generate an optical signal utilizing the laser, rotate the polarization of the optical signal utilizing the rotator, reflect the rotated optical signal onto the grating coupler on the die, and couple the optical signal to the waveguide, where an angle between a grating coupler axis that is parallel to the waveguide and a plane of incidence of the optical signal reflected to the angled grating coupler is non-zero. The angle between the grating coupler axis and the plane of incidence of the optical signal reflected to the angled grating coupler may be 45 degrees.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This application is a continuation-in-part of U.S. application Ser. No.14/324,544, filed on Jul. 7, 2014, which is a continuation ofapplication Ser. No. 13/894,052 filed on May 14, 2013, now U.S. Pat. No.8,772,704, which is a continuation of application Ser. No. 13/455,641filed on Apr. 25, 2012, now U.S. Pat. No. 8,440,989, which is acontinuation of application Ser. No. 12/500,465 filed on Jul. 9, 2009,now U.S. Pat. No. 8,168,939, which in turn makes reference to, claimspriority to and claims the benefit of U.S. Provisional PatentApplication No. 61/079,358 filed on Jul. 9, 2008. This applicationclaims priority to and the benefit of U.S. Provisional Application61/965,334 filed on Jan. 27, 2014, which is hereby incorporated hereinby reference in its entirety.

FIELD

Certain embodiments of the disclosure relate to semiconductorprocessing. More specifically, certain embodiments of the disclosurerelate to a method and system for coupling a light source assembly to anoptical integrated circuit.

BACKGROUND

As data networks scale to meet ever-increasing bandwidth requirements,the shortcomings of copper data channels are becoming apparent. Signalattenuation and crosstalk due to radiated electromagnetic energy are themain impediments encountered by designers of such systems. They can bemitigated to some extent with equalization, coding, and shielding, butthese techniques require considerable power, complexity, and cable bulkpenalties while offering only modest improvements in reach and verylimited scalability. Free of such channel limitations, opticalcommunication has been recognized as the successor to copper links.

Further limitations and disadvantages of conventional and traditionalapproaches will become apparent to one of skill in the art, throughcomparison of such systems with the present disclosure as set forth inthe remainder of the present application with reference to the drawings.

BRIEF SUMMARY

A system and/or method for coupling a light source assembly to anoptical integrated circuit, substantially as shown in and/or describedin connection with at least one of the figures, as set forth morecompletely in the claims.

Various advantages, aspects and novel features of the presentdisclosure, as well as details of an illustrated embodiment thereof,will be more fully understood from the following description anddrawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a block diagram of a photonically enabled CMOS chipcomprising angled grating couplers, in accordance with an exampleembodiment of the disclosure.

FIG. 1B is a diagram illustrating a CMOS chip, in accordance with anexample embodiment of the disclosure.

FIG. 1C is a diagram illustrating a CMOS chip coupled to an opticalfiber cable, in accordance with an example embodiment of the disclosure.

FIG. 2A illustrates a system coupling light from a laser chip to awaveguide on a transceiver chip, in accordance with an exampleembodiment of the disclosure.

FIG. 2B. Illustrates the polarization of light in a system with anisolator, in accordance with an example embodiment of the disclosure.

FIG. 3 illustrates side and top views of a light source assembly with agrating coupler, in accordance with an example embodiment of thedisclosure.

FIG. 4 illustrates a light source assembly without a polarizer, inaccordance with an example embodiment of the disclosure.

FIG. 5 illustrates a light source assembly with both non-reciprocal andreciprocal rotators, in accordance with an example embodiment of thedisclosure.

FIG. 6 illustrates the polarization direction after the individualoptical elements in the system of FIG. 5.

FIG. 7 illustrates a light source assembly without a reciprocal rotatorcoupling an optical signal into an angled grating coupler, in accordancewith an example embodiment of the disclosure.

FIG. 8 illustrates the polarization direction after the individualoptical elements in the system of FIG. 7 employing an angled gratingcoupler.

FIG. 9 illustrates wavevectors for grating coupler design, in accordancewith an example embodiment of the disclosure.

FIG. 10 illustrates a non-angled grating coupler in accordance with anexample embodiment of the disclosure.

FIG. 11 illustrates an angled grating coupler in accordance with anexample embodiment of the disclosure.

FIG. 12 illustrates a light source assembly and an angled gratingcoupler, in accordance with an example embodiment of the disclosure.

FIG. 13 illustrates a two output angled grating coupler, in accordancewith an example embodiment of the disclosure.

FIG. 14 illustrates a light source assembly with a reciprocal rotatorthat couples light to an angled grating coupler, in accordance with anexample embodiment of the disclosure.

FIG. 15 illustrates a light source assembly with a reciprocal rotator,and an angled grating coupler, in accordance with an example embodimentof the disclosure.

DETAILED DESCRIPTION

Certain aspects of the disclosure may be found in a method and systemfor coupling a light source assembly to an optical integrated circuit.Exemplary aspects of the disclosure may comprise a system comprising alaser source assembly having a laser, a rotator, and a mirror, where thelaser source assembly is coupled to a die comprising an angled gratingcoupler and a waveguide. The system may generate an optical signalutilizing the laser, rotate the polarization of the optical signalutilizing the rotator, reflect the rotated optical signal onto thegrating coupler on the die, and couple the optical signal to thewaveguide, where an angle between a grating coupler axis that isparallel to the waveguide and a plane of incidence of the optical signalreflected to the angled grating coupler is non-zero. The angle betweenthe grating coupler axis and the plane of incidence of the opticalsignal reflected to the angled grating coupler may be 45 degrees, forexample. The angled grating coupler may comprise grates with tangentialplanes at the grating coupler axis that are not perpendicular to thegrating coupler axis. The angle between the grating coupler axis and theplane of incidence of the optical signal reflected to the angled gratingcoupler may be configured by the rotator. The die may comprise a silicondie. The rotator may comprise a non-reciprocal rotator. The angledgrating coupler may comprise an overlay of two different angled gratingcouplers that couple signals into the waveguide and a second waveguideon the die. The optical signal reflected to the angled grating couplermay be split into the waveguide and the second waveguide utilizingoverlaid grating couplers. The rotator may comprise a reciprocalrotator. The laser may comprise a semiconductor laser.

FIG. 1A is a block diagram of a photonically enabled CMOS chipcomprising angled grating couplers, in accordance with an exampleembodiment of the disclosure. Referring to FIG. 1A, there is shownoptoelectronic devices on a CMOS chip 130 comprising optical modulators105A-105D, photodiodes 111A-111D, monitor photodiodes 113A-113H, andoptical devices comprising couplers 103A-103K, optical terminations115A-115D, and grating couplers 117A-117H. There are also shownelectrical devices and circuits comprising amplifiers 107A-107D, analogand digital control circuits 109, and control sections 112A-112D. Theamplifiers 107A-107D may comprise transimpedance and limiting amplifiers(TIA/LAs), for example.

Optical signals are communicated between optical and optoelectronicdevices via optical waveguides 110 fabricated in the CMOS chip 130.Single-mode or multi-mode waveguides may be used in photonic integratedcircuits. Single-mode operation enables direct connection to opticalsignal processing and networking elements. The term “single-mode” may beused for waveguides that support a single mode for each of the twopolarizations, transverse-electric (TE) and transverse-magnetic (TM), orfor waveguides that are truly single mode and only support one modewhose polarization is TE, which comprises an electric field parallel tothe substrate supporting the waveguides. Two typical waveguidecross-sections that are utilized comprise strip waveguides and ribwaveguides. Strip waveguides typically comprise a rectangularcross-section, whereas rib waveguides comprise a rib section on top of awaveguide slab.

The optical modulators 105A-105D comprise Mach-Zehnder or ringmodulators, for example, and enable the modulation of thecontinuous-wave (CW) laser input signal. The optical modulators105A-105D may comprise high-speed and low-speed phase modulationsections and are controlled by the control sections 112A-112D. Thehigh-speed phase modulation section of the optical modulators 105A-105Dmay modulate a CW light source signal with a data signal. The low-speedphase modulation section of the optical modulators 105A-105D maycompensate for slowly varying phase factors such as those induced bymismatch between the waveguides, waveguide temperature, or waveguidestress and is referred to as the passive phase, or the passive biasingof the MZI.

The outputs of the modulators 105A-105D may be optically coupled via thewaveguides 110 to the grating couplers 117E-117H. The couplers 103A-103Kmay comprise four-port optical couplers, for example, and may beutilized to sample or split the optical signals generated by the opticalmodulators 105A-105D, with the sampled signals being measured by themonitor photodiodes 113A-113H. The unused branches of the directionalcouplers 103D-103K may be terminated by optical terminations 115A-115Dto avoid back reflections of unwanted signals.

The grating couplers 117A-117H comprise optical gratings that enablecoupling of light into and out of the CMOS chip 130. The gratingcouplers 117A-117D may be utilized to couple light received from opticalfibers into the CMOS chip 130, and the grating couplers 117E-117H may beutilized to couple light from the CMOS chip 130 into optical fibers. Thegrating couplers 117A-117H may comprise single polarization gratingcouplers (SPCC) and/or polarization splitting grating couplers (PSCC).In instances where a PSCC is utilized, two input, or output, waveguidesmay be utilized.

The optical fibers may be epoxied, for example, to the CMOS chip, andmay be aligned at an angle from normal to the surface of the CMOS chip130 to optimize coupling efficiency. In an example embodiment, theoptical fibers may comprise single-mode fiber (SMF) and/orpolarization-maintaining fiber (PMF).

In another exemplary embodiment, optical signals may be communicateddirectly into the surface of the CMOS chip 130 without optical fibers bydirecting a light source on an optical coupling device in the chip, suchas the light source interface 135 and/or the optical fiber interface139. This may be accomplished with directed laser sources and/or opticalsources on another chip flip-chip bonded to the CMOS chip 130.

The photodiodes 111A-111D may convert optical signals received from thegrating couplers 117A-117D into electrical signals that are communicatedto the amplifiers 107A-107D for processing. In another embodiment of thedisclosure, the photodiodes 111A-111D may comprise high-speedheterojunction phototransistors, for example, and may comprise germanium(Ge) in the collector and base regions for absorption in the 1.3-1.6 μmoptical wavelength range, and may be integrated on a CMOSsilicon-on-insulator (SOI) wafer.

The analog and digital control circuits 109 may control gain levels orother parameters in the operation of the amplifiers 107A-107D, which maythen communicate electrical signals off the CMOS chip 130. The controlsections 112A-112D comprise electronic circuitry that enable modulationof the CW laser signal received from the splitters 103A-103C. Theoptical modulators 105A-105D may require high-speed electrical signalsto modulate the refractive index in respective branches of aMach-Zehnder interferometer (MZI), for example. In an exampleembodiment, the control sections 112A-112D may include sink and/orsource driver electronics that may enable a bidirectional link utilizinga single laser.

In operation, the CMOS chip 130 may be operable to transmit and/orreceive and process optical signals. Optical signals may be receivedfrom optical fibers by the grating couplers 117A-117D and converted toelectrical signals by the photodetectors 111A-111D. The electricalsignals may be amplified by transimpedance amplifiers in the amplifiers107A-107D, for example, and subsequently communicated to otherelectronic circuitry, not shown, in the CMOS chip 130.

An integrated transceiver may comprise at least three opticalinterfaces, including a transmitter input port to interface to the CWlight source, labeled as CW Laser In 101; a transmitter output port tointerface to the fiber carrying the optical signal, labeled OpticalSignals Out; and a receiver input port to interface to the fibercarrying the optical signal, labeled Optical Signals In.

Integrated photonics platforms allow the full functionality of anoptical transceiver to be integrated on a single chip. An opticaltransceiver chip contains optoelectronic circuits that create andprocess the optical/electrical signals on the transmitter (Tx) and thereceiver (Rx) sides, as well as optical interfaces that couple theoptical signals to and from a fiber. The signal processing functionalitymay include modulating the optical carrier, detecting the opticalsignal, splitting or combining data streams, and multiplexing ordemultiplexing data on carriers with different wavelengths.

It is often advantageous to have an external continuous-wave (CW) lightsource, because this architecture allows heat sinking and temperaturecontrol of the source separately from the transceiver chip 130. Anexternal light source may also be connected to the transceiver chip 130via a fiber interface. The light source can be integrated onto theintegrated optics chip in a hybrid fashion where a separately packagedlight source assembly is attached to the integrated optics chip.

The light source package may contain a lensing element to improvecoupling efficiency to the integrated optics chip, as well as anisolator to minimize reflections back to the laser chip. The isolatortypically comprises a non-reciprocal polarization rotator followed by apolarizer element. This isolator may be positioned between the couplingelement that couples the optical signal to an optical waveguide in theintegrated optics chip, as shown schematically in FIG. 2A. In an examplescenario, angled grating couplers may be utilized in the transceiverchip 130, which may reduce the rotator requirements of the light sourceassembly.

FIG. 1B is a diagram illustrating an exemplary CMOS chip, in accordancewith an exemplary embodiment of the disclosure. Referring to FIG. 1B,there is shown the CMOS chip 130 comprising electronic devices/circuits131, optical and optoelectronic devices 133, a light source interface135, CMOS chip front surface 137, an optical fiber interface 139, andCMOS guard ring 141.

The light source interface 135 and the optical fiber interface 139comprise grating couplers, for example, that enable coupling of lightsignals via the CMOS chip surface 137, as opposed to the edges of thechip as with conventional edge-emitting devices. Coupling light signalsvia the CMOS chip surface 137 enables the use of the CMOS guard ring 141which protects the chip mechanically and prevents the entry ofcontaminants via the chip edge.

The electronic devices/circuits 131 comprise circuitry such as theamplifiers 107A-107D and the analog and digital control circuits 109described with respect to FIG. 1A, for example. The optical andoptoelectronic devices 133 comprise devices such as the couplers103A-103K, optical terminations 115A-115D, grating couplers 117A-117H,optical modulators 105A-105D, high-speed heterojunction photodiodes111A-111D, and monitor photodiodes 113A-113H.

In an example scenario, the light source interface 135 may compriseangled grating couplers that select polarization near 45° with respectto the plane of incidence and is thus compatible with a light sourceassembly without a reciprocal rotator. This angled grating couplerdesign enables a simpler and cheaper to manufacture configuration, asdescribed further with respect to FIGS. 9, 11, 12, and, for example.

FIG. 1C is a diagram illustrating a CMOS chip coupled to an opticalfiber cable, in accordance with an exemplary embodiment of thedisclosure. Referring to FIG. 1C, there is shown the CMOS chip 130comprising the CMOS chip surface 137, and the CMOS guard ring 141. Thereis also shown a fiber-to-chip coupler 143, an optical fiber cable 145,and an optical source assembly 147.

The CMOS chip 130 comprising the electronic devices/circuits 131, theoptical and optoelectronic devices 133, the light source interface 135,the CMOS chip surface 137, and the CMOS guard ring 141 may be asdescribed with respect to FIG. 1B.

In an example embodiment, the optical fiber cable may be affixed, viaepoxy for example, to the CMOS chip surface 137. The fiber chip coupler143 enables the physical coupling of the optical fiber cable 145 to theCMOS chip 130.

In an example scenario, the light source interface 135 upon which thelight source module 147 is affixed may comprise angled grating couplersthat select polarization near 45° with respect to the plane ofincidence. Therefore, the light source module 147 may be configuredwithout a reciprocal rotator and still not suffer from back-reflections.This angled grating coupler design enables a simpler and cheaper tomanufacture configuration, as described further with respect to FIGS. 9,11, 12, and, for example.

FIG. 2A illustrates a system coupling light from a laser chip to awaveguide on a transceiver chip, in accordance with an exampleembodiment of the disclosure. Referring to FIG. 2A, there is shown acoupling system 200 comprising a laser 201, an isolator 210, a coupler207, and a waveguide 209. The laser 201 may be substantially similar tothe laser 101 described with respect to FIG. 1A, and may comprise acompound semiconductor laser chip, for example, that may be mountedwithin a light source assembly or module.

The light source assembly may also comprise the isolator 210, which maycomprise rotators 203A and 203B, and a polarizer 205. The rotators 203Amay comprise a non-reciprocal rotator, such as a Faraday rotator, forexample, and a reciprocal rotator. The combination of the rotators203A/B and the polarizer 205 may provide isolation from unwanted opticalreflections back to the laser 201, which can lead to output powerfluctuations.

FIG. 2B. Illustrates the polarization of light in a system with anisolator, in accordance with an example embodiment of the disclosure.The view shown in FIG. 2B is that from an observer directly behind thelaser 201, where the polarization of the optical signal transmitted fromthe laser is in the horizontal plane of the device, as is typical for asemiconductor laser. However, this is merely as an example to show howthe polarization changes as it passes through the rotator.

The rotator rotates the polarization of the light by 45° and the axis ofthe polarizer would then be oriented so that it allows light throughwhose polarization is along this direction. In this illustration, therotation is in the counter-clockwise direction; however, thepolarization may be rotated in the clockwise direction.

FIG. 3 illustrates side and top views of a light source assembly with agrating coupler, in accordance with an example embodiment of thedisclosure. An example of a hybrid light source that may be used inwhole or in part to support the light source assembly of the presentdisclosure, is described in U.S. Pat. No. 8,168,939, which is herebyincorporated by reference in its entirety. Referring to FIG. 3, there isshown a light source assembly 300 and a chip 320.

The light source assembly 300 may comprise a support substrate 301, alaser 303, a lens 305, a rotator 307, a lid 309, a mirror element 311,and a reciprocal rotator 313. The support substrate 301 may comprise asilicon optical bench, for example, that may support optical,electronic, and optoelectronic components and may be micro-machined outof a silicon substrate, for example. In an example scenario, thereciprocal rotator 313 may comprise a dielectric stack formed on thebottom of the substrate 301, and may comprise quartz, for example. Thereciprocal rotator 313 may be formed when the substrate 301 is still inwafer form, thereby reducing processing complexity and cost. The mirrorelement 311 may comprise a turning mirror and may be formed in the lid309, or may comprise a reflective structure affixed to the lid 309.

The lens 305 may comprise a spherical ball lens, for example, that maybe operable to focus light from the laser 303, and the rotator 307 maycomprise a Faraday rotator, for example, for rotating the polarizationof light focused by the lens 305. The laser 303 may comprise a compoundsemiconductor laser chip, for example, that may be mounted on a heatsink on the substrate 301.

The chip 320 may comprise a photonic or optoelectronic chip, such as asilicon CMOS photonics chip, for example, with an optoelectronictransceiver within which a grating coupler 321 and waveguide 323 may beformed. The grating coupler 321 may comprise an array of waveguidesand/or discrete scatterers that direct an optical signal received fromthe light source assembly 300 into the waveguide 323. The waveguide 323may comprise a higher dielectric constant material surrounded by lowerdielectric material, or air, that guides an optical signal along the topsurface of the chip 320.

In an example embodiment, the grating coupler 321 comprises apolarization-selective grating coupler. The turning mirror 311 in thelight source assembly 300 may project the polarized laser beam towardsthe chip 320 so that the beam directed onto the grating coupler 321 isclose to normal incidence to the chip. The grating coupler 321 maycouple the light into the waveguide 323 fabricated on the transceiverchip 320. FIG. 3 illustrates side and top views of the system, includingthe light source assembly and the grating coupler. The light signal path(drawn using a dashed line) defines the plane of incidence of the lightbeam.

In this configuration, the polarizer functionality may be provided bythe grating coupler itself, by virtue of its polarization selectivity.The reciprocal rotator 313 may rotate the polarization of the reflectedlight to orient it with the grating coupler 321. The grating coupler 321preferentially couples light polarized perpendicular to the plane ofincidence. In an example embodiment, the system comprising the lightsource assembly 300 and the grating coupler 321 would in principle allowthe removal of a separate polarizer element, as shown in FIG. 4,reducing complexity and cost.

FIG. 4 illustrates a light source assembly without a polarizer, inaccordance with an example embodiment of the disclosure. Referring toFIG. 4, there is shown a light source assembly 410 and a grating coupler420. In this example, the light source assembly comprises a laser 401, arotator 403, and a mirror 405, while the grating coupler 420 comprises apolarizer 407 and a coupler 409.

In this example, the grating coupler 420 itself provides the polarizerfunction in that only light that is polarized perpendicular to thegrating coupler axis, as shown in FIG. 3, is coupled to the waveguide411. As stated above, incorporating the polarizer functionality in thegrating coupler 420 reduces complexity and cost.

In practice, however, the polarization-selective grating coupler designthat preferentially couples light polarized perpendicular to the planeof incidence is incompatible with how the light is emitted from a lightsource assembly that only contains a lens, rotator, and mirror. Thisoccurs because the polarizer provided by the grating coupler is orientedperpendicular to the plane of incidence instead of 45° from it. For thisreason, a further element is added to the light source assembly, areciprocal rotator, as shown in FIG. 5, which corrects for the 45°polarization rotation affected by the non-reciprocal rotator.

FIG. 5 illustrates a light source assembly with both non-reciprocal andreciprocal rotators, in accordance with an example embodiment of thedisclosure. Referring to FIG. 5, there is shown a light source assembly510 and a grating coupler 520. In FIG. 5, the waveguide has been droppedfrom the figure for simplicity, but may share any and all aspects ofFIGS. 1A-4.

The light source assembly 510 comprises a laser 501, a non-reciprocalrotator 503, a mirror 505, and a reciprocal rotator 507. The gratingcoupler 520 may comprise a polarizer 509 and a coupler 511. To couple anoptical signal, existing polarization sensitive grating couplers receiveoptical signals at a polarization angle of 0°, meaning that anglebetween the grating coupler axis and the waveguide axis is zero and theangle between the optical signal polarization, i.e., the optical signalthat is coupled to the waveguide, and the grating coupler axis is 90°.

FIG. 6 illustrates the polarization direction after the individualoptical elements in the system of FIG. 5. Referring to FIG. 6, two viewsare shown: a back view, from the vantage point of an observer positionedbehind the laser, and a top view.

The polarization of the optical signal emitted from the laser 201 isshown by polarization (1), which is parallel to the horizontal plane inthe back view of FIG. 6. After the non-reciprocal rotator 503, thepolarization is rotated 45° in the vertical direction as shown bypolarization (2) in the back view of FIG. 6. Following the mirror 505 inthe light source assembly 510, the polarization of the optical signal isshown by the polarization (3) in the top view of FIG. 6. Finally, afterthe reciprocal rotator 507, which may comprise a dielectric stack formedin or on the substrate of FIG. 3, for example, the resultingpolarization is shown by (4) in the top view of FIG. 6.

In summary, even though the polarization-selective grating coupler 520allows the removal of the polarizer element, the reciprocal rotator 507is used to align the polarization of the optical signal with thepolarization axis of the grating coupler 520.

In an example scenario, an angled grating coupler that selectspolarization near 45° with respect to the plane of incidence maytherefore be compatible with a light source assembly without areciprocal rotator. This angled grating coupler design enables thesimpler and cheaper to manufacture configuration shown in FIG. 7

FIG. 7 illustrates a light source assembly without a reciprocal rotatorcoupling an optical signal into an angled grating coupler, in accordancewith an example embodiment of the disclosure. The example systemillustrated in FIG. 7 may, for example, share any or all functionalaspects discussed previously with regard to FIGS. 1A-6. Referring toFIG. 7, there is shown a light source assembly 710 and an angled gratingcoupler 720. The light source assembly 710 may be simplified frompreviously described assemblies as it comprises a laser 701, anon-reciprocal rotator 703, and a mirror 705. The angled grating coupler720 comprises a polarizer 707 with a polarization axis at 45° from theangle of incidence and a coupler 709.

The angled grating coupler 720 may comprise curved grates whosetangential planes at the grating coupler axis are not perpendicular tothe grating coupler axis. This is described further with respect toFIGS. 9 and 11, for example. Because the angled grating coupler 720couples light with an angle of incidence at 45° from the grating coupleraxis, a second rotator is not used.

FIG. 8 illustrates the polarization direction after the individualoptical elements in the system of FIG. 7 employing an angled gratingcoupler. Referring to FIG. 8, the polarization of the optical signalemitted from the laser 201 is shown by polarization (1), which isparallel to the horizontal plane in the back view of FIG. 8. After thenon-reciprocal rotator 703, the polarization is rotated 45° in thevertical direction as shown by polarization (2) in the back view of FIG.8. Following the mirror 705 in the light source assembly 710, thepolarization of the optical signal is shown by the polarization (3) inthe top view of FIG. 7.

In general, the angled grating coupler provides a way to couple anoptical signal to an integrated optics chip in the special case wherethe polarization of the light is not perpendicular to the plane ofincidence. Even though the particular example shown relates to couplinga light signal whose polarization is at 45° to the plane of incidence,the method is applicable to a system where this angle is arbitrary orotherwise different or determined.

FIG. 9 illustrates wavevectors for grating coupler design, in accordancewith an example embodiment of the disclosure. A grating coupler cantransform the free-space light beam emitted from a laser to the guidedmode in the waveguide on the transceiver chip using a diffractivegrating etched into the chip. In an example scenario, a non-angledgrating coupler comprises an array of etched linear or curved featuresthat are substantially perpendicular to the plane of incidence of thelaser light. To describe this design, the following definitions may beused:

θ=Incidence angle (angle between the normal to the chip and the lightbeam in the vicinity of the grating coupler)

k_(f)=Fiber mode wavevector

k_(g)=Waveguide mode wavevector

G=Reciprocal lattice vector of the locally periodic grating

λ=Free space wavelength of light emitted from the laser

n_(e)=Effective index of light propagation inside the grating

The light incident on the grating coupler may be focused to the entranceof the waveguide, which is shown in the figure as point P. Lightscattering is shown in FIG. 9 from point P₀ with r as the vector P₀P andφ as the angle.

The phase matching condition can be written as k_(g)=k_(f)+G, or,k _(g) ·{right arrow over (r)}−k _(f)·sin θ·{right arrow over (r)}=N·2πwhere N is an arbitrary integer.

This leads to the equation for a family of confocal ellipses with one ofits focal points at P:

$r = \frac{N\;\lambda_{e}}{1 - {e\;\cos\;\phi}}$where e is the eccentricity of the ellipses

$e = {\frac{n_{f}\sin\;\theta}{n_{e}}\mspace{14mu}{and}}$λ_(e) = λ/n_(e).

The grating is drawn along the ellipses (gray lines in the drawing) andthe individual grates correspond to different values of the integer N.The non-angled grating coupler is oriented in such a way that itssymmetry axis, i.e., the grating coupler axis, and the waveguide arealong the plane of incidence.

FIG. 10 illustrates a non-angled grating coupler in accordance with anexample embodiment of the disclosure. Referring to FIG. 10, there isshown a grating coupler 1000 and a waveguide 1003. There is also shown aplane of incidence 1005, grating coupler axis 1007, and a polarizationvector 1009. In existing grating couplers, the plane of incidence 1005coincides with the grating coupler axis 1007 so that optical signals canbe coupled into the waveguide, i.e., the polarization vector 1009 is 90°from the grating coupler axis 1007 where the waveguide 1003 extends outof the grating coupler 1000.

The grating coupler 1000 comprises an array of curved grates 1001, andas shown in FIG. 10, including in the inset that shows a magnified viewof the grating coupler 1000, the tangential planes of the grates 1001 atthe grating coupler axis 1007 are perpendicular to the grating coupleraxis 1007. Here, only one section of the ellipses that is near the planeof incidence is selected to draw the grating that couples light to thewaveguide. In this scenario, the plane of incidence 1005 is the same asthe grating coupler axis 1007, indicated by the 0° difference in anglebetween the plane of incidence 1005 and the grating coupler axis 1007.

FIG. 11 illustrates an angled grating coupler in accordance with anexample embodiment of the disclosure. Referring to FIG. 11, there isshown an angled grating coupler 1100 and a waveguide 1103. There is alsoshown a plane of incidence 1105, grating coupler axis 1107, and apolarization vector 1109. As shown in FIG. 11, the waveguide 1103 isstill oriented perpendicular to the polarization vector 1109, as wouldbe needed to couple an optical signal in to the waveguide 1103. However,in this example scenario, the grating coupler 1100 no longer has anexact axis of symmetry about the grating coupler axis 1107, even thoughthe axis of the grating coupler can be defined as the extension of thewaveguide.

The angled grating coupler 1100, in one embodiment, may be configured byselecting the portion of the ellipses of FIG. 9 that is not along theplane of incidence but is near the line that encloses an angle with itthat is substantially close to 45°. Accordingly, the plane of incidence1105 is at an angle of 45° from the grating coupler axis 1107.

Although a 45° example is shown, this method can be extended to designgrating couplers that accept light whose polarization is at an arbitraryor otherwise determined angle with respect to the plane of incidence,not only at 45°.

By decoupling the waveguide orientation from the plane of incidence, weobtain an optical element that can accept light whose polarization isnot necessarily perpendicular to the plane of incidence of the light.This angled grating coupler design can be used, for example, in theconfiguration shown in FIG. 12.

FIG. 12 illustrates a light source assembly and an angled gratingcoupler, in accordance with an example embodiment of the disclosure. Theexample system illustrated in FIG. 12 may, for example, share any or allfunctional aspects discussed previously with regard to FIGS. 1A-11.Referring to FIG. 12, there is shown a light source assembly 1210 and achip 1220 comprising a grating coupler 1221 and a waveguide 1223. Thelight source assembly 1210 may comprise a laser 1203, a lens 1205, and arotator 1207.

The lens 1205 may comprise a spherical ball lens, for example, and therotator 1207 may comprise a non-reciprocal rotator, such as a Faradayrotator, for example, for rotating the polarization of light focused bythe lens 1205.

The chip 1220 may comprise a photonic or optoelectronic chip, such as asilicon CMOS photonics chip, for example, with an optoelectronictransceiver within which a grating coupler 1221 and waveguide 1223 maybe formed. The grating coupler 1221 may comprise an array of waveguidesand/or discrete scatterers that direct an optical signal received fromthe light source assembly 1210 into the waveguide 1223. The waveguide1223 may comprise a higher dielectric constant material surrounded bylower dielectric material, or air, that guides an optical signal alongthe top surface of the chip 1220.

The turning mirror in the light source assembly 1210 may project therotated polarization laser beam towards the chip 1220 so that the beammay be directed onto the grating coupler 1221. The grating coupler 1221may couple the light into the waveguide 1223 fabricated on thetransceiver chip 1220. FIG. 12 illustrates side and top views of thesystem, including the light source assembly 1210 and the grating coupler1221. The light signal path (drawn using a dashed line) defines theplane of incidence of the light beam.

In an example embodiment, the grating coupler 1221 comprises an angledgrating coupler such that a second rotator is not needed in the lightsource assembly 1210, as is needed for existing grating couplers. Theangled grating coupler 1221 provides a way to couple an optical signalto an integrated optics chip in the special case where the polarizationof the light is not perpendicular to the plane of incidence. Even thoughin the particular example shown relates to coupling a light signal whosepolarization is at 45° to the plane of incidence, the method isapplicable to a system where this angle is arbitrary or otherwisedetermined.

FIG. 13 illustrates a two-output angled grating coupler, in accordancewith an example embodiment of the disclosure. The example systemillustrated in FIG. 13 may, for example, share any or all functionalaspects discussed previously with regard to FIGS. 1A-12. Referring toFIG. 13, the configuration shown may be similar to that shown in FIG.12, but with a two-output angled grating coupler 1321.

It should be noted that the optical signal from the light sourceassembly in the top view in FIG. 13 is shown slightly offset from thewaveguide 1323A for clarity, so as not to be confused with an opticalsignal coming into the grating coupler via the waveguide 1323A. As shownin the side view of FIG. 13, the optical signal from the light sourceassembly 1310 impinges on the two-output angled grating coupler 1321from the top.

In this example, two angled grating couplers may be overlaid to form atwo-dimensional grating, manufactured by etching a two-dimensionalpattern into the substrate on which the optical integrated circuit isformed, for example.

This type of grating coupler does not necessarily exhibit polarizationselectivity, but splits the optical power from the input optical signalinto two separate waveguides, in a ratio that is based on thepolarization of the incident light beam. The two-output angled gratingcoupler 1321 may be used in a parallel multi-channel transceiver, forexample, where one light source provides light for more than onechannel. In the particular case illustrated in FIG. 13, the axis of thegrating coupler is approximately at 45° to the plane of incidence andthe grating coupler splits the optical power approximately evenlybetween the two waveguides.

The two-output grating coupler 1321 may be based on the overlay of twodifferent angled grating couplers, one of which is designed for a 45°angle between the polarization vector and the plane of incidence, andthe other is designed for a 135°angle. This design is thus distinct frompolarization-splitting grating couplers where the plane of incidence isalong the plane of incidence. The example two-output grating coupler1321 shown in FIG. 13 also does not have an exact axis of symmetry.

As an added benefit, the reflection from an angled grating coupler backtowards the laser 1303 is reduced as compared to a non-angled gratingcoupler. One reason for this is that the grates of the grating couplerare perpendicular to the direction of the light propagation in thenon-angled case but are not so in the angled case. Therefore anypotential reflections from the grate closest to the waveguide will notpropagate back directly towards the laser but will be deflected by asmall angle.

This reduced reflection from the grating coupler may enable the removalof the rotator altogether because optical isolation may no longer benecessary to stabilize the laser power. Using a reciprocal rotator mayprovide a cost-advantage over the non-reciprocal rotator material, andalso a reduction in assembly cost, because typically the non-reciprocalrotator has to be poled using a high magnetic field to operate.

FIG. 14 illustrates a light source assembly with a reciprocal rotatorthat couples light to an angled grating coupler, in accordance with anexample embodiment of the disclosure. The example system illustrated inFIG. 14 may, for example, share any or all functional aspects discussedpreviously with regard to FIGS. 1A-13. Referring to FIG. 14, there isshown a light source assembly 1410 with the non-reciprocal rotatorremoved and replaced by a reciprocal rotator 1407. The reciprocalrotator 1407 enables the use of the angled grating coupler, which inturn reduces reflection back to the laser, which was the requirement forremoving the non-reciprocal rotator in the first place. Since isolationis not needed in this embodiment, the rotator need not rotate thepolarization by 45° but can instead rotate it by any angle, and theangled grating coupler can be designed accordingly.

FIG. 15 illustrates a light source assembly with a reciprocal rotator,and an angled grating coupler, in accordance with an example embodimentof the disclosure. The example system illustrated in FIG. 15 may, forexample, share any or all functional aspects discussed previously withregard to FIGS. 1A-14. Referring to FIG. 15, there is shown a lightsource assembly 1510 without a rotator in the substrate 1501 beyond thelens 1505 but instead a reciprocal rotator 1513 is formed on the bottomof the substrate. One advantage of this configuration could be that therotator could be bonded to the substrate in wafer form, and therefore itwould not have to be added individually for each laser assembly.

The reciprocal rotator 1513 enables the use of the angled gratingcoupler, which in turn reduces reflection back to the laser, which wasthe requirement for removing the non-reciprocal rotator in the firstplace. Since isolation is not needed in this embodiment, the rotatorneed not rotate the polarization by 45° but can instead rotate it by anyangle, and the angled grating coupler can be designed accordingly.

The disclosure is not restricted to the particular embodimentsdescribed, but the design principle can be extended to various types ofgrating couplers and light source assemblies, such as couplersmanufactured in various material platforms, couplers with apodizedgratings, couplers with grating curvatures that are not exactlydescribed by ellipses, couplers with substantially straight gratings,coupler whose waveguides are not at 45° to the plane of incidence, andother types of grating couplers not explicitly listed here.

In an example embodiment, a method and system are disclosed for couplinga light source assembly to an optical integrated circuit. In thisregard, aspects of the disclosure may comprise a system comprising alaser source assembly comprising a laser, a rotator, and a mirror, saidlaser source assembly coupled to a die comprising an angled gratingcoupler and a waveguide. An optical signal may be generated utilizingthe laser, the polarization of the optical signal may be rotatedutilizing the rotator, the rotated optical signal may be reflected ontothe grating coupler on the die, and the optical signal may be coupled tothe waveguide.

The angle between a grating coupler axis that is parallel to thewaveguide and a plane of incidence of the optical signal reflected tothe angled grating coupler is non-zero. The angle between the gratingcoupler axis and the plane of incidence of the optical signal reflectedto the angled grating coupler may be 45 degrees. The angled gratingcoupler may comprise grates with tangential planes at the gratingcoupler axis that are not perpendicular to the grating coupler axis.

The angle between the grating coupler axis and the plane of incidence ofthe optical signal reflected to the angled grating coupler may beconfigured by the rotator. The die may comprise a silicon die. Therotator may comprise a non-reciprocal rotator. The angled gratingcoupler may comprise an overlay of two different angled grating couplersthat couple signals into the waveguide and a second waveguide on thedie. The optical signal reflected to the angled grating coupler may besplit into the waveguide and the second waveguide utilizing overlaidgrating couplers. The rotator may comprise a reciprocal rotator. Thelaser may comprise a semiconductor laser.

As utilized herein the terms “circuits” and “circuitry” refer tophysical electronic components (i.e. hardware) and any software and/orfirmware (“code”) which may configure the hardware, be executed by thehardware, and or otherwise be associated with the hardware. As usedherein, for example, a particular processor and memory may comprise afirst “circuit” when executing a first one or more lines of code and maycomprise a second “circuit” when executing a second one or more lines ofcode. As utilized herein, “and/or” means any one or more of the items inthe list joined by “and/or”. As an example, “x and/or y” means anyelement of the three-element set {(x), (y), (x, y)}. In other words, “xand/or y” means “one or both of x and y”. As another example, “x, y,and/or z” means any element of the seven-element set {(x), (y), (z), (x,y), (x, z), (y, z), (x, y, z)}. In other words, “x, y and/or z” means“one or more of x, y and z”. As utilized herein, the term “exemplary”means serving as a non-limiting example, instance, or illustration. Asutilized herein, the terms “e.g.,” and “for example” set off lists ofone or more non-limiting examples, instances, or illustrations. Asutilized herein, circuitry is “operable” to perform a function wheneverthe circuitry comprises the necessary hardware and code (if any isnecessary) to perform the function, regardless of whether performance ofthe function is disabled or not enabled (e.g., by a user-configurablesetting, factory trim, etc.).

While the disclosure has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the present disclosure. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the present disclosure without departing from itsscope. Therefore, it is intended that the present disclosure not belimited to the particular embodiments disclosed, but that the presentdisclosure will include all embodiments falling within the scope of theappended claims.

What is claimed is:
 1. A method for communication, the methodcomprising: in a system comprising a laser source assembly comprising alaser, a rotator, and a mirror, said laser source assembly coupled to adie comprising an angled grating coupler and a waveguide: generating anoptical signal utilizing the laser; rotating the polarization of theoptical signal utilizing the rotator; coupling the optical signal to thegrating coupler in the die; and coupling the optical signal to thewaveguide utilizing the grating coupler, wherein an angle between agrating coupler axis that is parallel to the waveguide and a plane ofincidence of the optical signal reflected to the angled grating coupleris non-zero.
 2. The method according to claim 1, wherein the anglebetween the grating coupler axis and the plane of incidence of theoptical signal reflected to the angled grating coupler is 45 degrees. 3.The method according to claim 1, wherein the angled grating couplercomprises grates with tangential planes at the grating coupler axis thatare not perpendicular to the grating coupler axis.
 4. The methodaccording to claim 1, wherein the angle between the grating coupler axisand the plane of incidence of the optical signal reflected to the angledgrating coupler is configured by the rotator.
 5. The method according toclaim 1, wherein the rotator comprises a non-reciprocal rotator or areciprocal rotator.
 6. The method according to claim 1, wherein theangled grating coupler comprises an overlay of two different angledgrating couplers that couple signals into the waveguide and a secondwaveguide on the die.
 7. The method according to claim 6, comprisingsplitting the optical signal reflected to the angled grating couplerinto the waveguide and the second waveguide utilizing overlaid gratingcouplers.
 8. The method according to claim 1, wherein the rotator isformed at a bottom surface of the laser source assembly.
 9. The methodaccording to claim 1, comprising coupling the optical signal to thegrating coupler in the die utilizing a mirror element in a lid of thelaser source assembly.
 10. A system for communication, the systemcomprising: a laser source assembly comprising a laser, a rotator, and amirror, said laser source assembly coupled to a die comprising an angledgrating coupler and a waveguide, said system being operable to: generatean optical signal utilizing the laser; rotate the polarization of theoptical signal utilizing the rotator; couple the optical signal to thegrating coupler on the die; and couple the optical signal to thewaveguide, wherein an angle between a grating coupler axis that isparallel to the waveguide and a plane of incidence of the optical signalreflected to the angled grating coupler is non-zero.
 11. The systemaccording to claim 10, wherein the angle between the grating coupleraxis and the plane of incidence of the optical signal reflected to theangled grating coupler is 45 degrees.
 12. The system according to claim11, wherein the angled grating coupler comprises grates with tangentialplanes at the grating coupler axis that are not perpendicular to thegrating coupler axis.
 13. The system according to claim 11, wherein theangle between the grating coupler axis and the plane of incidence of theoptical signal reflected to the angled grating coupler is configured bythe rotator.
 14. The system according to claim 11, wherein the rotatorcomprises a non-reciprocal rotator or a reciprocal rotator.
 15. Thesystem according to claim 11, wherein the angled grating couplercomprises an overlay of two different angled grating couplers thatcouple signals into the waveguide and a second waveguide on the die. 16.The system according to claim 11, wherein the system is operable tosplit the optical signal reflected to the angled grating coupler intothe waveguide and the second waveguide utilizing the overlaid twodifferent grating couplers.
 17. The system according to claim 16,wherein the system is operable to couple the optical signal to thegrating coupler in the die utilizing a mirror element in a lid of thelaser source assembly.
 18. A semiconductor device comprising: a lasersource assembly comprising a laser, a rotator, and a mirror, said lasersource assembly coupled to a die comprising a grating coupler, saidgrating coupler comprising: an array of grates etched into a substrate;and a waveguide formed on said substrate, wherein a grating coupler axisof said grating coupler is parallel to said waveguide and said grateshave tangential planes at said grating coupler axis that are notperpendicular to said grating coupler axis, said semiconductor devicebeing operable to: generate an optical signal utilizing the laser;rotate the polarization of the optical signal utilizing the rotator;couple the optical signal to the grating coupler on the die; and couplethe optical signal to the waveguide.
 19. The semiconductor deviceaccording to claim 18, wherein the substrate is a silicon photonics die.20. The semiconductor device according to claim 18, wherein the gratingcoupler is operable to couple optical signals whose plane of incidenceis at a non-zero angle from the grating coupler axis.